Methods for producing aldehydes by the hydroformylation of an olefinically unsaturated organic compound with carbon monoxide and hydrogen (more commonly referred to as synthesis or syn gas) in the presence of a rhodium-phosphorus complex catalyst and free phosphorus ligand are well known in the art as seen; e.g., by the basic low pressure oxo hydroformylation process of U.S. Pat. No. 3,527,809 and the rhodium-catalyzed liquid recycle hydroformylation process of U.S. Pat. No. 4,148,830.
In U.S. Pat. No. 4,148,830 (the '830 Patent) it is disclosed that catalyst life can be enhanced and product yield improved by employing as a catalyst solvent for rhodium-catalyzed hydroformylation, higher boiling aldehyde condensation products (e.g. dimers, trimers and tetramers), as defined therein. It was also found that the rhodium catalyst could be continuously or intermittently recycled to the hydroformylation reaction zone without significant loss of rhodium, catalyst life, reaction rates and efficiency.
Accordingly, the '830 Patent disclosed that liquid effluent from the reaction zone containing catalyst, solvent and gases, is processed to strip and recover the aldehyde product. During this procedure some lights, e.g., hydrogen, carbon monoxide, unreacted olefin, corresponding alkane, and other by-product and inert gases dissolved in the reactor effluent are removed by reducing pressure on the effluent stream to flash off such gases. The desired aldehyde product is then recovered from the effluent and the remaining liquid residue fraction of unrecovered aldehydic product, catalyst and high boiling condensation product is recycled to the reactor. Thus, this process is often referred to as a liquid-recycle hydroformylation process (or "liquid recycle process").
Further to control the total reactor pressure in a liquid recycle process due to build up of inerts and the like, a gaseous purge is generally taken from the liquid recycle hydroformylation reactor, where excess hydrogen, carbon monoxide, unreacted olefin, inerts and alkane by-products, such as propane in the case of hydroformylating propylene, are vented as off-gases.
In addition, during the product separation step in a liquid recycle process, some gases, primarily unreacted olefin and alkane by-product, which remain dissolved in the liquid catalyst-containing effluent, are separated along with the desired aldehyde product. A portion of such separated gases are condensed with the desired aldehyde product. The remaining separated gases can be purged from the system.
Thus, the outright loss of unreacted olefin and syn gas components by purging in such liquid recycle processes could amount to a significant economic disadvantage over the life of commercial continuous hydroformylation operations designed to produce tens of millions of pounds of aldehyde per year, and various methods have been proposed to prevent this.
For instance, U.S. Pat. No. 4,593,127 illustrates a typical liquid recycle rhodium-catalyzed hydroformylation process as seen by the primary reactor system of FIG. 1 and the secondary reactor system of FIG. 2 of the drawings of said patent. Note that the vent gas line 28 in FIG. 1 that originates in the reactor headspace serves to conduct unreacted olefin and syn gas to the decoupled secondary reactor system of FIG. 1, while the gas vent lines 18 and 24 of said FIG. 1 each require compressors 20 and 21, if the unreacted olefin and syn gas are to be recycled to the hydroformylation reactor. On the other hand, the unreacted olefin and other light gases as seen by vent gas lines 91 and 92 of the secondary reactor system of FIG. 2 of said U.S. Pat. No 4,593,127 are given up as lost (e.g., burned as fuel). While it is taught that the vent gas line 91 may be recycled to the reactor, if desired, obviously such recycling will require the efforts of a compressor such as shown by compressors 20 and 21 in the primary reactor system of FIG. 1. Accordingly, while U.S. Pat. No 4,593,127 does teach the importance of recovering and recycling unreacted olefin from the vent gases of a liquid recycle hydroformylation process, such is taught to be accomplished by the employment of costly energy inefficient compressors and/or extensive additional processing equipment (e.g., the secondary reactor system of FIG. 1).
U.S. Pat. No. 4,210,426, discloses recovering propene and propane from the off-gas of a process involving the hydroformylation of propene by contacting the off-gas with a liquid absorbent comprising at least one of the liquid substances formed during the hydroformylation, e.g., n-and/or iso-butyraldehyde, to absorb the propene and propane and thereafter subsequently desorbing these gases from the absorbent by lowering the pressure and/or raising the temperature. The propene and propane may then be separated by distillation. Alternatively, it is possible to separate off only the propene from the absorbent by combining the desorption with fractional distillation and the propane subsequently removed from the absorbent by distillation. Preferably the propene is recycled to the reactor and the absorbent to the absorption column. Note, however, that the recovery system of U.S. Pat. No. 4,210,466 promotes the employment of high absorption pressures (column 2, lines 4-33) and illustrates the need for extensive processing equipment (three independent recovery columns).
U.S. application Ser. No. 370,806 filed June 23, 1989, now U.S. Pat. No 5,001,274 patent Mar. 19, 1991, discloses recovering absorbable gaseous components, such as unreacted olefin and product aldehyde, contained, e.g., in the vent gas of the reactor of liquid recycle rhodium-catalyzed hydroformylation process, by contacting said vent gas with the hydroformylation catalyst solution that is being recycled to the reactor after its separation from the product aldehyde, so as to absorb said gaseous components therein. The resulting catalyst solution is then returned directly to the hydroformylation reactor without any desorption or separation of the gaseous unreacted olefin and product aldehyde. However, as in any system that employs the rhodium-catalyst solution that is being recycled to the reactor there is always a danger of losing rhodium, e.g., via the purge lines 28, 31a or 31b from the scrubbers shown in the drawing of said U.S. Pat. No. 5,001,274.
U.S. Pat. No. 4,827,043 discloses purifying gas streams, e.g., syn gas, by contacting them with at least a portion of the aldehyde containing product stream obtained from a gas recycle hydroformylation process prior to introducing the syn gas feed into the hydroformylation reaction zone. The process is said to remove undesirable impurities from the syn gas, e.g., oxygen, sulfur bearing lubricating oil and iron carbonyl. Said patent also discloses (Example 5) recovering and recycling unreacted propylene from crude hydroformylation aldehyde product streams, in the same manner as disclosed in INDICATIONS, Winter 1982/83, the International Journal of Davy McKee, pp. 20 to 28 published by the public affairs department of the Davy Corporation, London, England.